Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (“RF”) frequency spectrum.
In order to properly interpret the signal, conventional RF receivers extract the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry. In order to perform this extraction, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (“IF”) signal. This IF signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called “heterodyne” receiver architectures. Naturally, circuit elements (called “IF components”) are required in order to deal with the intermediate conversion to and from the intermediate frequency.
It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. One technology developed in order to reduce RF receiver cost, size, and power consumption is generally termed direct conversion. Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often also called zero-IF, synchrodyne, or homodyne receiver architectures.
FIG. 1 is an illustration of a conventional direct conversion circuit 100 in accordance with the prior art. With reference to FIG. 1, the circuit 100 includes an antenna 101 which receives the RF modulated signal. The antenna 101 then provides the received signal to an amplifier 102 which amplifies the signal for further processing. The amplifier 102 may be, for example, an RF low noise amplifier. The amplified signal is then split into two branches, an in-phase (“I”) branch 110, and a quadrature-phase (“Q”) branch 120. Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch 110 includes an in-phase mixer 111, and the quadrature-phase branch 120 includes a quadrature-phase mixer 121. A local oscillator 130 generally provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a ninety degree phase shifter 131 which causes the control signal for the quadrature-phase mixer 121 to be ninety degrees out of phase with the control signal for the in-phase mixer 111. The signal from the in-phase mixer 111 is then passed through a low pass filter 112 to a baseband amplifier 113 to complete the extraction of the baseband signal from the received signal as far as the in-phase branch 110 is concerned. Likewise, the signal from the quadrature-phase mixer 121 is passed through a low pass filter 122 to a baseband amplifier 123 to complete the extraction of the baseband signal as far as the quadrature-phase branch is concerned. The in-phase and quadrature-phase baseband signals are then processed by signal processing circuitry 150.
A conventional intermediate conversion circuit in accordance with the prior art would be similar to the direct conversion circuit of FIG. 1 with the addition of IF components to convert the received modulated RF signal through an intermediate frequency. While direct conversion circuits may generally be smaller and require less power than conventional heterodyne receiver architectures, direct conversion architectures characteristically introduces more DC offset and 1/f noise than do heterodyne receiver architectures thereby limiting dynamic range.
Furthermore, it is often desirable to implement antenna diversity in any receiver architecture. Antenna diversity involves the use of more than one antenna to receive a signal to improve the ability to properly receive the signal. When using one antenna to receive a signal, the signal may have actually taken several paths from the transmitter to the receiver, each having a different length. This causes an echo effect that might actually lead to destructive interference between the signals receive from different paths. The use of two or more antennas that are appropriately spaced reduces the degradation due to the echo effect since the echo at one antenna will typically be different than the echo at another, thereby reducing the likelihood that the echo would degrade the signal.
Thus, a continuing need exists for an improved receiver architecture, particularly for microwave and millimeter wave systems operating in higher frequency ranges. As greater uses are made for microwave and millimeter wave radio systems, it would be advantageous if a unique receiver architecture could be found that addressed the low cost and performance requirements in the area of millimeter wave and microwave communications systems without degradation to the technical performance of a respective transceiver, transmitter, or receiver and/or communication system.